Here's something that playing a lot of the recently released "Just Cause 2" got me thinking about. I have profoundly enjoyed nearly every open-world action game I have played, but one or two exceptionally annoying aspects have prevented each of them from reaching "all-time favorite" status. I have a dream that one day, an open-world game will come along that gets all of the following things right:

DO: Give players the means to navigate the world in a reasonably quick manner without a vehicle. We've all been there before: the vehicle you just had control of five seconds ago catches on fire from one too many dinks and scrapes, forcing you to bail out and abandon your only means of speedy transportation in the middle of nowhere. When running toward the nearest sign of civilization for a car ends up saving you time compared to directly heading to your destination on foot, your game is doing something very wrong.
Prime example: "Just Cause 2"
Worst offender: "Grand Theft Auto IV"

DON'T: Fill your world with hundreds of items to collect unless players can locate them easily. The sensation of wonderment and empowerment you get from picking up collectibles eventually turns to horror and frustration as those collectibles become increasingly sparse. Without a radar or some other form of hint system, wandering 10 square miles of space for the last five trinkets is not my idea of a good time. Teasing players with an achievement for collecting 100 percent of them makes it even worse!
Prime example: "Infamous"
Worst offender: "Crackdown"

DO: Make every inch of your world interesting to look at. I can hear the developers now: "Oh, sure, we'll make our artists give up their nights and weekends to do just that. Thanks for the tip, jackass." I know, it's a logistical nightmare, and not every game has the multi-million dollar budget of "Grand Theft Auto IV." But that game does exist, and therefore the audience's standards have changed. Gamers will know if you got lazy and copy-pasted textures and environments just to pad out the size of your world.
Prime example: "Grand Theft Auto IV"
Worst offender: "Red Faction: Guerilla"

DON'T: Create carbon copies of missions that differ only in their story context. This is an unfortunate byproduct of the player's abilities being finite, so having each mission be completely unique is kind of an impossible dream for guys aiming to create a lengthy, multi-hour open-world experience. Then again, no one said every open-world game has to push forty hours and offer hundreds of missions. Quality over quantity.
Worst Offenders: "Brütal Legend," "Burnout Paradise," "Infamous"